Here's a quick guide on how armor works and best stats on each piece of gear in Tom Clancy's: The Division.

What does Armor Does:

The more armor you have, the more you benefit from it. Looking at multiple points you get a static 1% DR for 71.9 armor (at lvl 30). Going from 20% Armor to 30% armor is an effective Damage Reduction (DR) of 1- .7/.8 = .125 = 12.5% Going from 55% armor to 65% armor (cap) is an effective DR of 1 - .35/.45 = .222 = 22.2% Not only that, but as you gain more armor the ratio of 1% DR per armor drops from 71.9 down to closer to 71, making it even stronger.

Armor mitigates GUN damage - if you are pumped with a grenade, there is no hope for you.

Where to get armor from:

All gear has built in armor bonuses. However, the following gear can roll +armor on them:

Body Armor ( largest bonus)

Back Pack

Knee Pads

Holster

Low Armor:

If you have all purple armor, with no +armor bonus, you'll sit somewhere between 35% and 38-39% damage reduction

Medium Armor:

If you have all purples with 1-2 armor bonuses, you'll sit between 40-49% damage reduction - this really depends which gear you have the mods on, as chest pieces have more bonus armor

If you have all purples with 2-4 +armor bonuses, you'll sit between 48-53-60% damage reduction.

With chest / knee pads / backpack with +armor I was able to get 3709 armor, and 51.96% damage reduction

So how much armor do you need?

It is easy for most players to land in the medium armor range and benefit from the solid 40-49% gun damage reduction. Just two decently rolled pieces of gear can easily get you to 45%+

To get into the high tier armor bracket, you'll need 3-4 +armor pieces. You shouldn't be eating damage if you plan on doing damage, so don't waste stats on armor.

Best Stats Per Gear Piece

Potential Stat Rolls + Ranking (Only listing top tier stats)

Body Armor (3 bonuses allowed):

1. Armor

2. Damage to Elites

3. Gear mod slot

4. Protection from Elites

Body armor doesn't have life changing potential bonuses - Because of this, I'd suggest on your chest piece to grab:

+Armour

+Elite Damage

Your choice of mod slot / health on kill / protection from elites

Back Pack (1 primary stat)

Critical hit damage

Armor

Highly recommend taking critical hit damage, as only 3 pieces of gear can get it. If you are low armor however, you may need the bonus armor to survive.

Knee Pads (two bonuses allowed):

Critical Damage

Armor

Damage to Elites

Gear Mod

Protection from elites

Knee pads allow for bonus critical hit damage, one of the best stats in the game for DPS.

Tankier Build: Armor + Critical Hit damage

DPS Build: Damage to Elites + Critical Hit damage

Holster (one bonus)

Armor

Gear mod

Protection from elites

This order is is pretty well the same for what you should pick.

Need armor? +Armor

Need health? Stamina gear mod

Need damage? Firearm mod

Gloves ( 3 bonus DPS KINGS):

Critical Hit Damage

Critical Hit Chance

+ Weapon Base Damage

+Damage to Elites

Gloves are one of the biggest boosts to your overall damage in the game. Take +Weapon damage for PVP and +Elite damage for PVE content

Masks (two bonuses):

Critical hit chance

+Damage to elites

Gear Mod Slot

Masks are one of the only slots that provide bonus critical hit chance. Please get it haha.

If you run an smg build, you can focus on a gear mod slot and bonus elite damage...but crit chance is always amazing.

recap

For gear that you can feel alright getting +armor on:

Chest

Knee pads

For DPS:

All other gear should focus on damage dealing bonuses such as critical hit damage, critical hit chance, and bonus damage to elites

For Tanking:

More armor can be chosen, +health bonuses, and mod slots.

Notice:

These stats are pretty well universally "valuable" in any MMO / Loot Based game

Additional Notes:

"Considering you get 7 gear mod slots, each can land 140~ stats and a random major attribute including 1.5% crit chance. By not prioritizing mod slots on each gear piece, you literally will lose out on 1000~ stat points and 11%~ crit chance. That is the difference between 150k DPS and 200k, while keeping your HP relatively the same. TL;DR mod slots are absolutely the best major attributes on gear slots, and should always be prioritized over other major attributes unless for very specific builds. This of course is just my opinion :)" - This does require nearly perfect rolls on your mods that go inside your mod slots, but still a valid point

Fo you prefer watching than reading, then check out the video below which basically explains what was written above.